1. Field of the Invention
The present invention relates to optical disk units, and more particularly to an optical disk unit for recording information on and reproducing information from DRAW-type (DRAW: Direct Read After Write) optical disks such as CD-R, CD-RW, DVD-R, DVD-RW, and DVD+RW disks.
2. Description of the Related Art
Data recording on DRAW-type optical disks such as CD-R, CD-RW, DVD-R, DVD-RW, and DVD+RW disks should be completed in a single writing operation. A writing method for such a writing operation is called “Track at Once” or “Disk at Once.” If data is written to an optical disk at a higher rate than that at which the data is transferred from a host computer, thus resulting in data shortage in the middle of the writing operation, that is, if buffer under run occurs in the middle of the writing operation, the writing operation to the optical disk results in failure.
The buffer under run is more likely to occur as the data writing rate becomes higher. Once the buffer under run occurs, it may cause fatal damage to a recording medium that is not rewritable, such as a CD-R disk. Conventionally, the occurrence of buffer under run is avoided by increasing the capacity of a buffer RAM (formed of, for instance, a DRAM) or making the data writing rate lower than the data transfer rate.
Japanese Laid-Open Patent Application No. 10-49990 discloses a technology not for avoiding the occurrence of buffer under run but for coping with the buffer under run by suspending data writing to an optical disk temporarily and restarting the data writing after a sufficient amount of data is transferred from a host computer to be stored in a buffer RAM in the case of the occurrence of buffer under run. Thereby, the data writing can be completed normally without failure even in the case of the occurrence of buffer under run, which could otherwise be fatal to the data writing.
According to the recording control disclosed in the above-described patent application, data recording on the optical disk may be suspended and restarted frequently. Normally, data is transferred from the host computer to be stored temporarily in the buffer RAM, so that a predetermined parity is encoded and added to the data. Thereafter, the data is subjected to cross-interleave Reed-Solomon code (CIRC) coding and eight to fourteen modulation (EFM) to be recorded on the optical disk as recording data. At present, the buffer RAMs for the conventional optical disk units range from 5 Kbytes to 8 Mbytes in capacity.
On the other hand, the recording rate for the CDs, for instance, which is 150 Kbytes per second at normal speed, is as high as 2.4 Mbytes per second at 16× speed. Therefore, with the recording rate becoming higher at the current pace, all of the data temporarily stored in the buffer RAM is recorded on the optical disk in a very short period of time. That is, if there is no transfer of new data from the host computer to the buffer RAM during this limited period of time, the buffer under run occurs so that the recording operation is suspended. Although the host computer connected to the optical disk unit boasts better performance, the optical disk unit should be prepared for frequent suspension and restarting of its recording operation so as to ensure stable operation to any host computer.
Under the condition where the recording operation is frequently suspended and restarted, however, if the recording operation is controlled so that recording is restarted precisely from a position at which the recording was previously ended or suspended by detecting the position, and if there is a difference between absolute position information and the position of the recorded data on the optical disk, the difference accumulates to exceed a certain amount so that the recording may be prevented from being restarted. That is, as previously described, the maximum amount of data that is transferred from the host computer in a single transmission and can be stored in the buffer RAM is equivalent to the capacity of the buffer RAM. Thus, the recording operation should be suspended if there is no new data transfer from the host computer before recording of the data stored in the buffer RAM is completed. The possibility of the occurrence of this problem increases as the buffer RAM is reduced in capacity and/or the rate of recording data on the optical disk increases.